In general the present invention relates to calenders for fiber web in a fiber web machine. More especially the present invention relates to a method for modernizing a multiroll calender for a fiber web in which at least one of the calender rolls is a smooth-surfaced press-roll.
Calendering is generally carried out in order to improve the properties, like smoothness and gloss, of a web-like material such as a paper or board web. In calendering the web is passed into a calendering nip formed between rolls that are pressed against each other, in which nip the web becomes deformed as by the action of temperature, moisture and nip pressure. In the calender the nips are formed between a smooth-surfaced press roll such as a metal roll and a roll coated with resilient material such as a polymer roll. The resilient surfaced roll adjusts itself to the forms of the web surface and presses the opposite side of the web evenly against the smooth-surfaced press roll. Multinip or multiroll calendering is calendering in a calendering unit in which the nips are formed between a smooth-surfaced press roll such as a metal roll and a roll coated with resilient material such as a polymer roll and the linear load increases in multinip calenders from the upper nip to the lower nip due to gravitation, unless roll relief systems are used.
From the prior art multiroll calenders are known in which a set of rolls formed of two stacks of rolls, with each stack of rolls comprising at least two calender rolls. Multinip or multiroll calender are known for example by the brand names OptiLoad® and TwinLine™. In F1 patent 96334 is disclosed a method for calendering a paper or an equivalent web material in a calender where the web to be calendered is passed through nips formed by a deflection-compensated upper roll and a deflection-compensated lower roll, and by two or more intermediate rolls arranged between the upper and lower roll. The rolls are arranged as a substantially vertical stack of rolls. Supercalenders are multiroll calenders composed of alternating hard and soft rolls, which soft rolls are filled rolls that have a steel shaft around which a multiplicity of specialty paper sheets with a hole in the middle are slid. The paper sheets are then compressed with a hydraulic press. The most common number of rolls is 9-12, but here can be up to 16 rolls.
In publication F1 20095284 is disclosed a heatable and/or coolable roll for fiber web machines which comprises a mantle part and end pieces attached to the ends of the mantle part for arranging a feed channel and a discharge channel for a heat transfer medium in the roll. The mantle part comprises flow channels for the heat transfer medium. The mantle part of the roll comprises an inner layer and an outer layer attached on the inner layer and the flow channels for heat transfer medium comprise flow grooves formed on the outer surface of the inner layer, and the roll comprises at its first end a first circumferential flow chamber, which is in flow connection with the feed channel or the discharge channel for the heat transfer medium of the roll, wherein the roll comprises first flow bushings, which are arranged in connection with the first flow chamber such that by each first flow bushing is formed a flow connection for the heat transfer medium between the first circumferential flow chamber and the flow channel of the mantle part. Flow channels can be arranged to form a spiral shaped geometry in the mantle, thus minimizing vibration excitation from uneven thermal expansion from fluid channels.
As fiber web machine running speeds increase the need for increasing calendering capacity increases. Also the need for increasing capacity of existing calenders increases since in many prior art fiber web machines the calender, in particular the supercalender, has become the part of the fiber web production line that limits the available capacity and hinders the production of full capacity of the other parts of the fiber web production line. Due to this, in many cases two or even three supercalenders are needed for calendering the production of the fiber web production line.
The line between paper and board is flexible and paper and board are available in a wide variety of grades and can be divided according to basis weight into two categories: papers with a single ply and a basis weight of 25-300 g/m2 and boards manufactured in multi-ply technology and having a basis weight of 150-600 g/m2. It should be noted that the border line between paper and board is flexible since board grades with lightest basis weights are lighter than the heaviest paper grades. Generally speaking, paper is used for printing and board for packaging.
The subsequent descriptions are examples of some values presently applied for fibrous webs, and there may be considerable variations from the disclosed values. The descriptions are mainly based on the source publication Papermaking Science and Technology, section Papermaking Part 3, edited by Jokio, M., published by Fapet Oy, Jyvaskyla 1999, page 362.
Mechanical-pulp based, i.e. wood-containing printing papers include newsprint, uncoated magazine and coated magazine paper.
Newsprint is composed either completely of mechanical pulp or may contain some bleached softwood pulp (0-15%) and/or recycled fiber to replace some of the mechanical pulp. General values for newsprint can be regarded as follows: basis weight 40-48.8 g/m2, ash content (SCAN-P 5:63) 0-20%, PPS s10 roughness (SCAN-P 76:95) 3.0-4.5 pm, Bendtsen roughness (SCAN-P 21:67) 100-200 ml/min, density 200-750 kg/m3, brightness (ISO 2470:1999) 57-63%, and opacity (ISO 2470:1998) 90-96%.
Uncoated magazine paper (SC=supercalendered) usually contains mechanical pulp to 50-70%, bleached softwood pulp to 10-25%, and fillers to 15-30%. Typical values for calendered SC paper (containing e.g. SC-C, SC-B. SC-NM-) include basis weight 40-60 g/m2, ash content (SCAN-P 5:63) 0-35%, Hunter gloss (ISO/DIS 8254/1)<20-50%, PPS s10 roughness (SCAN-P 76:95) 1.2-2.5 pm, Bendtsen roughness (SCAN-P 21:67) 100-200 ml/min, density 700-1250 kg/m3, brightness (ISO 2470:1999) 62-70%, and opacity (ISO 2470:1998) 90-95%.
Coated magazine paper (LWC=light weight coated) contains mechanical pulp to 40-60%, bleached softwood pulp to 25-40%, and fillers and coaters to 20-35%. General values for LWC paper can be regarded as follows: basis weight 40-70 g/m2, Hunter gloss 50-65%, PPS s10 roughness 0.8-1.5 pm (offset), 0.6-1.0 pm (roto), density 1100-1250 kg/m3, brightness 70-75%, and opacity 89-94%.
General values for MFC (machine finished coated) can be regarded as follows: basis weight 50-70 g/m2, Hunter gloss 25-70%, PPS s10 roughness 2.2-2.8 pm, density 900-950 kg/m3, brightness 70-75%, and opacity 91-95%.
General values for FCO (film coated offset) can be regarded as follows: basis weight 40-70 g/m2, Hunter gloss 45-55%, PPS s10 roughness 1.5-2.0 pm, density 1000-1050 kg/m3, brightness 70-75%, and opacity 91-95%.
General values for MWC (medium weight coated) can be regarded as follows: basis weight 70-90 g/m3, Hunter gloss 65-75%, PPS s10 roughness 0.6-1.0 pm, density 1150-1250 kg/m3, brightness 70-75%, and opacity 89-94%.
HWC (heavy weight coated) has a basis weight of 100-135 g/m2 and can be coated even more than twice.
Pulp-produced, wood free printing papers or fine papers include uncoated—and coated—pulp-based printing papers, in which the portion of mechanical pulp is less than 10%.
Uncoated pulp-based printing papers (WFU) contain bleached birchwood pulp to 55-80%, bleached softwood pulp 0-30%, and fillers to 10-30%. The values with WFU have a large variation: basis weight 50-90 g/m2, Bendtsen roughness 250-400 ml/min, brightness 86-92%, and opacity 83-98%.
In coated pulp-based printing papers (WFC), the amounts of coating vary widely in accordance with requirements and intended application. The following are typical values for once and twice-coated, pulp-based printing paper: once-coated basis weight 90 g/m2, Hunter gloss 65-80%, PPS s10 roughness 0.75-2.2 pm, brightness 80-88%, and opacity 91-94%, and twice-coated basis weight 130 g/m2, Hunter gloss 70-80%, PPS s10 roughness 0.65-0.95 pm, brightness 83-90%, and opacity 95-97%.
Release papers have a basis weight within the range of 25-150 g/m2.
Other papers include e.g. sackkraft papers, tissues, and wallpaper bases.
Board making makes use of chemical pulp, mechanical pulp and/or recycled pulp. Boards can be divided e.g. in the following main groups according to applications thereof: Corrugated board, comprising a liner and fluting; Boxboards, used for making boxes, cases, which boxboards include e.g. liquid packaging boards (FBB=folding boxboard, LPB=liquid packaging board, WLC=white-lined chipboard, SBS=solid bleached sulfite, SUS=solid unbleached sulfite); Graphic boards, used for making e.g. cards, files, folders, cases, covers, etc. and wallpaper bases.
An object of the present invention is to create a method for modernizing a multiroll calender and a modernized multiroll calender in which high capacity of the production line is achieved.
Another object of the present invention is to achieve a method and a multiroll calender in which possible disadvantages and problems of known methods and calenders are eliminated or at least minimized.